Saturn's Moons Engage in Cosmic Paintball Fight

by SPACE.com Staff | October 13, 2010 07:45am ET

These three views of Saturn's moon Rhea were made from data obtained by NASA’s Cassini spacecraft, enhanced to show colorful splotches and bands on the icy moon’s surface.Credit: NASA/JPL/SSI/LPI [Full Story]

Five of Saturn's inner moons are engaged in a cosmic
paintball fight, pelting each other with particles that leave bright, colorful
splotches and may be the source of a Pac-man feature on one of the satellites.

NASA's Cassini
spacecraft recently spotted reddish and blue splashes on the icy surfaces
of Mimas, Enceladus, Tethys, Dione and Rhea. The colors are helping scientists
map out how material travels between these moons of the ringed planet, and
they're highlighting how "space weathering" can impact bodies in our
solar system, researchers said. [New photo of colorful Saturn moon.]

The discovery may also help explain the mysterious Pac-Man
heat pattern
Cassini spotted on Mimas earlier this year, scientists said.

"The beauty of it all is how the satellites behave as a
family, recording similar processes and events on their surfaces, each in its
own unique way," said study lead author Paul Schenk, of the Lunar and
Planetary Institute in Houston, in a statement. "I don't think anyone
expected that electrons would leave such obvious fingerprints on planetary
surfaces, but we see it on several moons, including Mimas, which was once
thought to be rather bland."

The research is detailed in a recent online edition of the
science journal Icarus.

Enceladus: The aggressor

In the new study, Schenk and his colleagues used raw images
taken by Cassini from 2004 to 2009, to produce new, high-resolution global
color maps of the five Saturn moons.

The chief aggressor in the Saturnian particle paint war
appears to be icy Enceladus, researchers said.

Mysterious
ice geysers blast from the south polar region of this small moon. Particles
from these icy jets make up most of Saturn's misty E ring, and they also appear
to splatter Tethys, Dione and Rhea.

These three moons run head-on into Enceladus' spray as they
orbit Saturn, and the gunk leaves a coral-colored stain on their icy surfaces,
researchers said.

Enceladus' spray also tags Mimas, but it hits the moon's
trailing side. This probably occurs because Mimas orbits inside the path of
Enceladus ? or closer to Saturn ? while Tethys, Dione and Rhea are on the
outside, scientists said.

But Enceladus doesn't emerge from the paintball fight
unscathed: Some of its own icy material blows back onto the moon, dyeing parts
of its surface bright blue.

Scientists aren't sure why the frost stains the other moons
pinkish and Enceladus blue, but they're working on figuring it out.

Mimas' Pac-Man pattern

On Tethys, Dione and Rhea, darker, rusty reds paint the entire trailing
hemisphere ? the side that faces backward in the orbit around Saturn,
scientists said. These colors are thought to be caused by tiny particle strikes
from circulating plasma ? an electrically charged form of matter similar to gas
? in Saturn's magnetic environment.

In addition, Mimas and Tethys both sport a dark bluish band. The bands match
patterns that might be produced if the surface were being irradiated by
high-energy electrons drifting in a direction opposite to the flow of plasma in
the magnetic bubble around Saturn, researchers said.

Scientists are still figuring out exactly what is happening,
but the electrons appear to be zapping the Mimas surface in a way that matches
the Pac-Man thermal pattern detected by Cassini's composite infrared
spectrometer, Schenk said.

Schenk and his colleagues also found a unique chain of bluish splotches along
the equator of Rhea that re-opens the question of whether Rhea ever had a ring
around it. The splotches do not seem related to Enceladus, but rather appear
where fresh, bluish ice has been exposed on older crater rims.

Though scientists recently reported that they did not see
evidence in Cassini images of a ring around Rhea, the authors of the new study
suggest the crash of orbiting material ? perhaps a ring ? to the surface of
Rhea in the not-too-distant past could explain the bluish splotches.

"Analyzing the image color ratios is a great way to really enhance the
otherwise subtle color variations and make apparent some of the processes at
play in the Saturn
system," said Amanda Hendrix, Cassini deputy project scientist at
NASA's Jet Propulsion Laboratory, Pasadena, Calif. "The Cassini images
highlight the importance and potential effects of so-called 'space weathering'
that occurs throughout the solar system ? on any surface that isn't protected
by a thick atmosphere or magnetic field."